1. Field of the Invention
The present invention relates to an exposure apparatus which exposes a substrate to pulsed light supplied from a light source, and a method of manufacturing a device using the same.
2. Description of the Related Art
An exposure apparatus is employed in lithography for manufacturing a device such as a semiconductor device. The exposure apparatus illuminates an original by an illumination optical system with light supplied from a light source to project the pattern of the original onto a substrate via a projection optical system, thereby exposing the substrate.
The resolution of the exposure apparatus is known to be proportional to a wavelength λ of light emitted by the light source, and inversely proportional to a numerical aperture NA of the projection optical system as given by:resolution=k1·(λ/NA)  (1)where k1 is a proportionality constant.
To improve the resolution of the exposure apparatus, it is only necessary to shorten the wavelength of light emitted by the light source or increase the numerical aperture of the projection optical system.
One characteristic of the projection optical system of the exposure apparatus is the depth of focus (DOF). The depth of focus expresses the allowable range of blurring of an image projected onto the substrate by the distance from the focal point. The depth of focus can be expressed by:DOF=k2·(λ/NA2)  (2)where k2 is a proportionality constant.
As the wavelength of light emitted by the light source is shortened or the numerical aperture of the projection optical system is increased in order to improve the resolution of the exposure apparatus, the depth of focus decreases accordingly. Such a decrease in depth of focus is a serious problem especially for the next-generation devices being developed aiming at higher packing densities by the miniaturization and three-dimensional design of circuit patterns. That is, because the three-dimensional design of a circuit pattern amounts to increasing the dimension of the formed structure in the optical axis direction, a decrease in depth of focus disturbs the three-dimensional design of a circuit pattern.
To solve the above-described problem, an attempt has been made to increase the depth of focus by imaging an original pattern at a plurality of positions on the same optical axis with exposure light containing a plurality of wavelength components.
Japanese Patent Registration No. 02619473 discloses a technique of providing a light source for emitting a light beam with a first wavelength and a light source for emitting a light beam with a second wavelength, and combining these light beams, thereby using the combined light as exposure light.
Japanese Patent Laid-Open No. 11-162824 discloses a technique of inserting a filter for selectively transmitting light beams in a plurality of wavelength ranges into the optical path between the light source and the substrate, thereby using the transmitted light as exposure light with a plurality of wavelength components.
Japanese Patent Laid-Open No. 06-252021 discloses a technique of changing the wavelength of light emitted by the light source during the cumulative exposure of the substrate to pulsed light. FIGS. 1 and 2 are graphs each illustrating the spectral distribution of pulsed light in cumulative exposure.
In recent years, it has become possible to periodically change the wavelength between −Δλ and +Δλ at high speed. However, Japanese Patent Laid-Open No. 06-252021 neither discloses nor suggests details of how to determine the changing period of the wavelength of pulsed light emitted by the light source.
If the wavelength changing period is inappropriate, it is impossible to obtain a spectral distribution with a high symmetry as shown in FIGS. 1 and 2 in cumulative exposure with a plurality of pulsed light beams. In this case, the best focus position is offset in the optical axis direction, resulting in a decrease in depth of focus.